With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among them, a Wireless Local Area Network (WLAN) is a technique for wirelessly accessing the Internet at homes or companies or in specific service providing areas by using portable terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), based on wireless frequency technology.
A lot of standardization tasks are being performed since Institute of Electrical and Electronics Engineering (IEEE) 802 (i.e., the standardization organization of WLAN technology) was established on February, 1980. WLAN technology initially supported a speed of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a frequency of 2.4 GHz according to IEEE 802.11, but recently may support a maximum speed of 54 Mbps by using Orthogonal Frequency Division Multiplexing (OFDM). In addition, in IEEE 802.11, standardizations for various techniques, such as the improvement of Quality of Service (QoS), Access Point (AP) protocol compatibility, security enhancement, radio resource measurement, wireless access vehicular environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, are being put to practical use or developed.
A basic access mechanism of IEEE 802.11 Medium Access Control (MAC) is a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism associated with binary exponential back-off. The CSMA/CA mechanism is also called a Distributed Coordination Function (DCF) of IEEE 802.11 MAC, and it basically adopts a “listen before talk” access mechanism. In this type of an access mechanism, a Station (STA) listens to a wireless channel or medium prior to transmission. If, as a result of the listening, the medium is detected as not being used, the listening STA starts its own transmission. If, as a result of the listening, the medium is detected as being used, however, the listening STA does not start its own transmission and enters a delay period which is determined by the binary exponential back-off algorithm.
The CSMA/CA mechanism also includes virtual carrier sensing in addition to physical carrier sensing where an STA directly listens to a medium. The virtual carrier sensing is for supplementing the limits of the physical carrier sensing, such as a hidden node problem. For the virtual carrier sensing, IEEE 802.11 MAC uses a Network Allocation Vector (NAV). The NAV is a value that enables an STA, now using a medium or having a right to use the medium, to indicate the time remaining until the medium is available for another STA. Accordingly, the value of the NAV corresponds to the period where the use of a medium is reserved by an STA that transmits a relevant frame.
One of procedures of setting the NAV is a procedure of exchanging a Request To Send (RTS) frame and a Clear To Send (CTS) frame. The RTS frame and the CTS frame include information capable of delaying the transmission of frames by reception STAs by informing the reception STAs of upcoming frame transmission. The information may be included in, for example, the duration fields of the RTS frame and the CTS frame. After the RTS frame and the CTS frame are exchanged, a source STA transmits an actual frame to a destination STA.
The CSMA/CA-based channel access method, however, has a problem in that efficiency is not high. For example, if a PHY Service Access Point (SAP) provides a throughput of 1 Gbps, a MAC SAP may provide only about 50 to 60% of the throughput of 1 Gbps provided by the PHY SAP. Furthermore, if a plurality of STAs is associated with one Access Point (AP), overall efficiency of a WLAN system may be deteriorated. An individual STA has to access a channel through contention and may find it further difficult to obtain an opportunity to send a radio frame using a Wireless Medium (WM).